periscope

Internal optics of a periscope.

Periscope in use.

A periscope is an optical instrument that permits an observer to view his surroundings
along a displaced axis, and hence from a concealed, protected, or submerged
position. The simplest periscope, used in tanks, has two parallel reflecting
surfaces (prisms and mirrors).
An auxiliary telescopic gunsight may be added. Submarine telescopes have
a series of lenses within the tube to widen
the field of view, cross-wires, and
a range-finder, and can rotate and retract.

I n its simplest form, a periscope consists of two mirrors angled at 45°,
one above the other so that an image is reflected from the top of the instrument
down to the observer at its base. A submarine periscope (shown in the photo
below and the diagram to the right) works on the same principle, but has
prisms instead of mirrors and a system of lenses to produce a magnified
image or a wide field of view. Optical adjustments can be made with the
handles.